BEIJING (AFP) - A detained Chinese activist has managed to release a video from behind bars urging citizens to fight for their rights in an act likely to further antagonize authorities. 

Xu Zhiyong, who has been detained since mid-July, is among two dozen activists rounded up this year for criticizing corruption and other sensitive topics -- despite a government campaign to fight graft. 

"Let us come together and work to make the rights of citizens’ matter, the identities of citizens’ matter, and together push for democracy, rule of law, fairness and justice," he said.

"We can surely build a free, public-spirited, loving and good China." 

Xu wore handcuffs and sat behind metal bars in a quiet setting in the 80-second video posted on Thursday by the University of Hong Kong's China Media Project. 

It is not known where Xu is detained.

The ruling Communist Party clamps down on those urging democracy or fighting for individual rights for fear social unrest could challenge its grip on power. 

Xu was detained for "gathering a crowd to disrupt public order,” the US-based Human Rights in China group said.

He had signed an open letter in April urging the release of activists who had called for officials to disclose their assets, it said.

Altogether 25 activists, lawyers and others have been detained since February for speaking out against official corruption and "promoting other politically 'sensitive' goals,” the US-based China Human Rights Defenders said last month.